NEURONAL CELL TYPES
Local interneurons and projection neurons demonstrate characteristic size, dendritic arborizations, and axonal projections. In the CNS (denoted by dashed lines), glial cells (astrocytes, microglia, oligodendroglia) provide support, protection, and maintenance of neurons. Schwann cells and satellite cells provide these functions in the PNS.
Neuronal form and configuration provide evidence of the role of that particular type of neuron. Dorsal root ganglion cells have virtually no synapses on the cell body; the sensory receptor is contiguous with the initial segment of the axon to permit direct activation of the initial segment upon reaching a threshold stimulus. This arrangement provides virtually no opportunity for centrifugal control of the initial sensory input; rather, control and analysis of the sensory input occurs in the CNS. Purkinje neurons in the cerebellum have huge planar dendritic trees, with activation occurring via hundreds of parallel fibers and the background excitability influenced by climbing fiber control. This type of array allows network modulation of Purkinje cell output, via neurons of the deep cerebellar nuclei, to UMNs, a control mechanism that permits fine-grained, ongoing adjustments to smooth and coordinated motor activities. Small interneurons in many regions have local and specialized functions that have local circuit connections, whereas large isodendritic neurons of the reticular formation receive widespread, polymodal, nonlocal input, which is important for general arousal of the cerebral cortex and consciousness. Damage to these key neurons may result in coma. LMNs and preganglionic autonomic neurons receive tremendous convergence upon their dendrites and cell bodies to orchestrate the final pattern of activation of these final common pathway neurons through which the peripheral effector tissues are signaled and through which all behavior is achieved.